1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for deghosting seismic data acquired with a variable-depth streamer.
2. Discussion of the Background
Marine seismic data acquisition and processing generate an image of a geophysical structure (subsurface) under the seafloor. While this image/profile does not provide a precise location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of oil and/or gas reservoirs. Thus, providing a high-resolution image of the subsurface is an ongoing process for the exploration of natural resources.
During a seismic gathering process, as shown in FIG. 1, a vessel 10 tows plural detectors 12 disposed along a cable 14. Cable 14 together with its corresponding detectors 12 are sometimes referred to by those skilled in the art as a streamer 16. The vessel 10 may tow plural streamers 16 simultaneously. The streamers may be disposed horizontally, i.e., lying at a constant depth z1 relative to the surface 18 of the ocean. Also, the plural streamers 16 may form a constant angle (i.e., the streamers may be slanted) with respect to the surface of the ocean as disclosed in U.S. Pat. No. 4,992,992, the entire content of which is incorporated herein by reference. FIG. 2 shows such a configuration in which all the detectors 12 are distributed along a slanted straight line 14 that makes a constant angle α with a reference horizontal line 30.
With reference to FIG. 1, the vessel 10 also tows a seismic source 20 configured to generate an acoustic wave 22a. Acoustic wave 22a propagates downward and penetrates the seafloor 24, eventually being reflected by a reflecting structure 26 (reflector). Reflected acoustic wave 22b propagates upward and is detected by detector 12. For simplicity, FIG. 1 shows only two paths 22a corresponding to the acoustic wave. However, the acoustic wave emitted by source 20 may be substantially a spherical wave, e.g., it propagates in all directions starting from the source 20. Some of reflected acoustic waves 22b (primary) are recorded by the various detectors 12 (the recorded signals are called traces) while some reflected waves 22c pass detectors 12 and arrive at the water surface 18. Because the interface between the water and air is well approximated as a quasi-perfect reflector (i.e., the water surface acts as a mirror for the acoustic waves), reflected wave 22c is reflected back toward detector 12 as shown by wave 22d in FIG. 1. Wave 22d is normally referred to as a ghost wave because it is due to a spurious reflection. Ghosts are also recorded by detector 12, but with a reverse polarity and a time lag relative to primary wave 22b. The degenerative effect the ghost arrival has on seismic bandwidth and resolution is known. In essence, interference between primary and ghost arrivals causes notches, or gaps, in the frequency content the detectors record.
The traces may be used to determine the subsurface (i.e., earth structure below surface 24) and to determine the position and presence of reflectors 26. However, ghosts disturb the accuracy of the final image of the subsurface and, for at least this reason, various methods exist for removing ghosts, i.e., deghosting, from the results of a seismic analysis.
However, most existing methods are designed for handling seismic data recorded with horizontal streamers, i.e., seismic data collected at the same depth (datum) relative to the ocean surface. Recent developments require that processing methods handle seismic data collected with curved and/or slanted streamers, i.e., seismic data collected by receivers located at variable depths.
Accordingly, it would be desirable to provide systems and methods for 3D seismic processing which allow imaging of the subsurface geology based on marine seismic data recorded at different water depths.